The use of an expansive area change immediately following the diaphragm has afforded considerable convenience in shock tube applications and has been studied by a number of authors. It allows the use of a smaller diaphragm, thereby reducing the disturbance to the shock tube flow caused by the diaphragm opening, and it also permits greater flexibility in the choice of driver section configurations for a given shock tube size.

It is natural to consider the extension of this idea to shock tunnel operation. This has been encouraged by the possibility of applying area change to operation of an arc-driven shock-tunnel. Some early success in employing arc-heated drivers for shock tunnels has been reported by Warren et al. However, the configuration they studied employed a long driver section to achieve correspondingly long test times. In addition, to achieve a constant shock velocity it was necesssary that electrical energy was added so that the gas was heated uniformly before the diaphragm ruptured. This necessitated a fast discharge condenser bank, with a high discharge voltage, the latter to enable an arc to be sustained over the length of the driver section. In the present application, a constriction between the driver section and the shock tube is employed, allowing the driver section to be treated as a constant pressure reservoir of gas. It is anticipated that this will allow the use of shorter drivers, thereby permitting the adoption of lower discharge voltages, and will also allow energy to be added relatively slowly while the shock tunnel is operating. This will produce substantial economies in provision of the large energy storage facilities required for shock-tunnel operation.